This invention relates to power semiconductor devices, the hermetical seal of which is improved.
Power semiconductor devices for rectifiers or the like used for fork lifts and electric cars generally have high current capacities and, therefore, require large element areas. Thermal fatigue of soldered portions of electrodes and breakage of bonding wires due to fusion are common occurrences in such devices. Desirably, these devices are made without resorting to soldering or bonding. In addition, these devices should desirably have a flat top surface for various parts which have to be mounted thereon. Furthermore, a hermetic seal is required to prevent water from intruding into the inner element section when washing the vehicle.
A highly reliable and safe design is thus required for the power semiconductor device.
To meet these requirements, there has been proposed a construction as shown in FIGS. 1 and 2. The construction is free from soldering and bonding and uses a spring member which is held in a compression bonded state. It has a flat top surface, and an electrode is adapted to be taken out in a lateral direction. Furthermore, a hermetic seal of the inside is ensured by using O-rings. More particularly, referring to FIGS. 1 and 2, an aluminum support plate 2, which also serves as a heat radiator, supports a compression bonding type semiconductor element 4, such as a diode, such that its anode electrode 3 is in contact with the top of the support plate 2. The aluminum support plate 2 also supports a case 5 made of a resin such that it is open at both its upper and lower ends. The case 5 surrounds the semiconductor element 4. A cover plate 6 seals the top opening of the case 5. The aluminum support plate 2, case 5 and cover plate 6 are secured to one another by suitable securing means such as a plurality of screws 7. At least either one of the confronting surfaces of the case 5 and cover plate 6, for instance the confronting surface of the case 5, is provided with a groove surrounding the open top. An O-ring 9, which is made of an elastic insulator, is received in the groove 8 and compressed by the cover plate 6. The interior of the case 5 is thus hermetically sealed.
The aluminum support plate 2 is provided with recesses 10 so that the screws 7 will not project from the support plate 2. An electrode body 13 is disposed over the cathode electrode 11 of the semiconductor element 4. It is mounted on the semiconductor element via a metal support 12 made of molybdenum or tungsten having substantially the same coefficient of thermal expansion as that of the semiconductor material, for instance silicon, in order to prevent cracking of the element 3 due to thermal fatigue.
On the electrode body 13 is provided one end of a cathode lead electrode 15, which has its other end extending through a hole 14 formed in a side wall of the case 5. On the end portion of the cathode lead electrode 15 over the semiconductor element are mounted, via an insulator 16 and a washer 17, a plurality of dish springs 18. The dish springs 18 are urged toward the semiconductor element 4. That is, the semiconductor element 4 and cathode lead electrode 15 are held against the aluminum support plate 2 by the dish springs 18. Extending centrally of the dish springs 18 is a guide 19 for guiding the dish springs. An L-shaped cathode lead electrode 20 is secured by a screw 21 to the outer side face of the cathode lead electrode 15. At the outer end of the hole 14, at least one of the confronting surfaces of the electrode 15 and case 5, for instance the confronting surface of the case 5, is provided with a groove 22 surrounding the electrode 15, and an O-ring is received in the groove 22 in an urged state and compressed therein, whereby the hermetical seal of the interior of the case 5 is ensured.
With this power semiconductor device, a predetermined pressure is applied to the cathode lead electrode 15 and semiconductor element 4 via the dish springs 18 while also urging the O-rings 9 and 23, thus maintaining the hermetic seal of the interior of the case 5 to protect the semiconductor element 4.
With this construction, however, when a predetermined pressure is applied to the semiconductor element 4, the deformation of the O-rings 9 and 23 is unbalanced. This results from the cumulative tolerances of the various parts supported in a stacked state on the aluminum support plate 2. Therefore, a predetermined pressure cannot be uniformly applied to the O-rings 9 and 23, so that the hermetic seal cannot be steadily ensured.